1. Field of the Invention
The present invention relates to a battery, and more particularly, to a secondary battery with a pouch casing having an improved reinforcement structure.
2. Discussion of the Related Art
Rechargeable secondary batteries are typically small in size and have a large storage capacity. Some examples of the batteries include nickel-metal hydride (Ni—MH) and lithium (Li) ion batteries.
Secondary batteries may be categorized as cylindrical, rectangular, and pouch type. The cylindrical batteries may use an aluminum can type structure, while the rectangular batteries typically use a rectangular aluminum can type structure. The pouch type batteries may be arranged in a thin film pouch casing.
FIG. 1 is an extracted perspective view of a pouch type secondary battery according to the related art.
Referring to FIG. 1, an electrode assembly 10 for generating a predetermined current and a pouch casing 20 for enclosing and hermetically sealing the electrode assembly 10. The electrode assembly 10 has an electrode tab 14 arranged on a side of the assembly. The electrode tab 14 is connected with a lead 15 for an electrical path to the exterior of the battery. The lead 15 has a protective tape 16 adhered to its surface and extends outside the pouch casing 20.
The pouch casing 20 includes a casing body 22. The casing body 22 has an internal space in which the electrode assembly 10 is retained. A cover 24 extends from one end of the casing body 22. A flange 23 is formed at the periphery of the casing body 22. The flange 23 may be welded with the cover 24 for creating a hermetic seal with the casing body 22. The pouch casing 20 is made of metal and resin materials in the form of a foil material.
FIG. 2 is a cross-sectional view taken along the line I-I of FIG. 1 according to the related art.
Referring to FIG. 2, the pouch casing 20 is formed of compressed composite materials. For example, a laminate structure of a thermal adhesion layer 22b, a foil layer 22a, another thermal adhesion layer 22b, and a sheathing layer 22c. The thermal adhesion layer 22b may be formed of denatured polypropylene, for example, cast polypropylene (CPP). The foil 22a may be made of a metal, for example, aluminum (Al). The sheathing 22c may be formed of a polymer resin, for example, nylon or polyethyleneterephthalate (PET). The pouch casing 20 may be sealed via heat compression, thereby joining the thermal adhesion layers of the casing body 22 and the pouch cover 24.
The above-described configuration is very thin and provides a poor protection for the electrode assembly housed in the pouch casing 20. In particular, the metal foil used for maintaining the strength of the casing is only several tens of microns thick. Accordingly, it is very weak against external impacts.
Also, the safety and reliability of the battery may be impaired if the related art pouch is disturbed.
The melting point of the thermal adhesion layer (e.g., made of CPP) of the related art pouch is about 130 to 140° C. The melting point of the electrode assembly housed in the pouch casing is also about 130 to 140° C. The temperature required to thermally weld or seal the casing body and pouch cover is about 180 to 210° C. Accordingly, during the welding of the pouch the thermal adhesion layer and the separator may melt. Also, the separator may be fixed to the thermal adhesion layer due to the high temperatures. If the thermal adhesion layer melts the Al metal film inside may be exposed, and thus creating short circuit of the electrode assembly.
FIG. 3 is a cross-sectional view illustrating a bi-cell stacked structure of an electrode assembly according to the related art.
Referring to FIG. 3, illustrating a laminate construction of positive electrode sheets 11 and negative electrode sheets 12 separated by separators 13. That is, a laminate of having a positive electrode sheet 11, a separator sheet 13, a negative electrode sheet 12, another separator sheet 13, and another positive electrode sheet. The area of each of the separator sheets 13 is greater than the area of the individual positive electrode sheet 11 and negative electrode sheet 12. Additionally, the area of the positive electrode sheets 11 is the smallest. The sheets are arranged to form the electrode assembly 10.
A swelling phenomenon occurs inside the electrode assembly 10. That is, gas may be generated inside the battery as the battery is repeatedly charged and discharged, thereby the electrode sheets may swell in the direction of the arrows as shown in FIG. 3. Additionally, the separator sheets 13 may shrink compounding the problem. As a result the positive electrode sheet 11 and the negative electrode sheet 12 may become short circuited with each other at edges.
Related art solutions to protect the battery from short circuits will be described in the following.
By way of example, U.S. Pat. No. 6,042,966 discloses a battery structure for preventing electrical shorts between an electrode tab and an edge of a pouch. The pouch includes an outer lamination layer made of a packaging insulator film, a metal foil layer, and an inner lamination layer having a polyamide layer. The inner lamination layer includes, for example, nylon, and a thermal adhesion layer sequentially formed on the inner face of the metal foil.
U.S. Pat. No. 6,106,973 discloses a pouch formed by folding a laminated film including an aluminum (Al) film at the inner side and a polypropylene film at the outer side and arranged on both sides of an electrode assembly.
The above-described battery pouches have several drawbacks including poor structural stability. Also, a stacked electrode assembly may cause a short circuit between electrodes.
Accordingly, there is a need for battery apparatus that has good structural stability and one that minimizes shorting. The present invention obviates one or more of the problems associated with related art.